LoadVoice: Fix phonemes_name initialization

phonemes_name is only initialized when V_LANGUAGE is met. This is not
necessarily the case, notably with

testing espeak_SetVoiceByName("!v/Annie") (language variant; intonation)
Cannot set intonation: language not set, or is invalid.
Uninitialized bytes in __interceptor_strcmp at offset 0 inside [0x7fff8a875e30, 1)
==4169902==WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 0x4c6a49 in LookupPhonemeTable /home/samy/brl/speech/espeak-ng-git/src/libespeak-ng/synthdata.c:363:7
    #1 0x4c6a49 in SelectPhonemeTableName /home/samy/brl/speech/espeak-ng-git/src/libespeak-ng/synthdata.c:380:12
    #2 0x5098a9 in LoadVoice /home/samy/brl/speech/espeak-ng-git/src/libespeak-ng/voices.c:950:34
    #3 0x50edcf in espeak_ng_SetVoiceByName /home/samy/brl/speech/espeak-ng-git/src/libespeak-ng/voices.c:1585:7
    #4 0x4aad63 in espeak_SetVoiceByName /home/samy/brl/speech/espeak-ng-git/src/libespeak-ng/espeak_api.c:125:32
    #5 0x4a3fe1 in test_espeak_set_voice_by_name_language_variant_intonation_parameter /home/samy/brl/speech/espeak-ng-git/tests/api.c:356:2
    #6 0x4a3fe1 in main /home/samy/brl/speech/espeak-ng-git/tests/api.c:567:2
    #7 0x7f26e88cb7fc in __libc_start_main csu/../csu/libc-start.c:332:16
    #8 0x4213a9 in _start (/home/samy/ens/projet/1/speech/espeak-ng-git/tests/api.test+0x4213a9)

  Uninitialized value was created by an allocation of 'phonemes_name' in the stack frame of function 'LoadVoice'
    #0 0x504290 in LoadVoice /home/samy/brl/speech/espeak-ng-git/src/libespeak-ng/voices.c:519

so better catch it properly rather than relying on uninitialized data.

src/libespeak-ng/voices.c

 	char language_name[40];
 	char translator_name[40];
 	char new_dictionary[40];
-	char phonemes_name[40];
+	char phonemes_name[40] = "";
 	const char *language_type;
 	char buf[sizeof(path_home)+30];
 	char path_voices[sizeof(path_home)+12];